1. Field of the Invention
The present invention relates to a lens control apparatus and a lens control method as well as a storage medium, all of which are suitable for use in a camera provided with an inner focus type of lens system.
2. Description of Related Art
FIG. 1 is a view showing a simple arrangement of an inner focus type of lens system which has conventionally been used. The arrangement shown in FIG. 1 includes a fixed first lens group 101, a second lens group (variator lens) 102 for performing a magnification varying operation, an iris 103, a fixed third lens group 104, a fourth lens group (hereinafter referred to as the focusing lens) 105 having both a focus adjusting function and a so-called compensation function which compensates for a movement of a focal plane due to a magnification varying operation, and a CCD (image pickup element) 106.
In the lens system which is arranged as shown in FIG. 1, since the focusing lens 105 has both the compensation function and the focus adjusting function, the position of the focusing lens 105 for forming an in-focus image on an image pickup surface of the CCD 106 differs for different subject distances even in the case of the same focal length.
If a variation in the position of the focusing lens 105 for forming an in-focus image on the image pickup surface of the CCD 106 is continuously plotted against different subject distances for different focal lengths, the resultant loci are as shown in FIG. 2. During a magnification varying operation, zooming free of defocusing is enabled by selecting a locus from the loci shown in FIG. 2 according to the subject distance and moving the focusing lens 105 along the selected locus.
A front-lens focus type of lens system is provided with a compensator lens which is independent of a variator lens, and the variator lens and the compensator lens are connected to each other by a mechanical cam ring. Accordingly, if a knob for manual zooming is provided on the cam ring so that the focal length can be manually varied, no matter how fast the knob may be moved, the cam ring rotates in accordance with the movement of the knob, and the variator lens and the compensator lens move along a cam groove in the cam ring. Therefore, as long as the focusing lens is in focus, the above operation does not cause defocusing.
In the control of the inner focus type of lens system having the above-described feature, it is general practice to previously store a plurality of pieces of locus information such as those shown in FIG. 2 in a lens control microcomputer in a particular form, select a locus according to the relative position between the focusing lens and the variator lens, and perform zooming while tracing the selected locus.
In such control, it is necessary to read the position of each of the focusing lens and the variator lens with a certain degree of accuracy, because the position of the focusing lens relative to the position of the variator lens is read from a storage element and applied to lens control. As can be seen from FIG. 2 as well, if the variator lens moves at or near a uniform speed, the inclination of the locus of the focusing lens successively varies with a variation in the focal length. This indicates that the moving speed and direction of the focusing lens vary successively. In other words, an actuator for the focusing lens needs to make a highly accurate speed response of 1 Hz up to several hundred Hz.
It is becoming general practice to use a stepping motor for the focusing lens of the inner focus type of lens system as an actuator which satisfies the above requirement. The stepping motor is capable of rotating in complete synchronism with a step pulse outputted from a lens control microcomputer or the like and showing a constant stepping angle per pulse, so that the stepping motor can realize high speed response, high stopping accuracy and high positional accuracy.
Furthermore, the stepping motor provides the advantage that since its rotating angle per step pulse is constant, the step pulse can be used for an increment type of encoder and a special position encoder is not needed.
As described above, if a magnification varying operation is to be carried out while keeping an in-focus state by using such a stepping motor, it is necessary to previously store the locus information shown in FIG. 2 in the lens control microcomputer or the like in a particular form (the loci themselves may be stored or a function which uses lens positions as variables may be stored), and read locus information according to the position or the moving speed of the variator lens and move the focusing lens on the basis of the read locus information.
FIGS. 3(a) and 3(b) are views aiding in explaining a locus tracing method which has previously been devised. In FIG. 3(a), z0, z1, z2, . . . , z6 indicate the position of the variator lens, a0, a1, a2, . . . , a6 and b0, b1, b2, . . . , b6 respectively indicate representative loci stored in the lens control microcomputer, and p0, p1, p2, . . . , p6 respectively indicate positions on a locus calculated on the basis of the two loci. An equation for calculating this locus is shown below: EQU p(n+1)=(.vertline.p(n)-a(n).vertline./.vertline.b(n)-a(n).vertline.)* .vertline.b(n+1)-a(n+1).vertline.+a(n+1). (1)
According to Equation (1), for example, if the focusing lens is located at the position p0 in FIG. 3(a), the ratio in which the position p0 internally divides a line segment b0-a0 is obtained, and a point which internally divides a line segment b1-a1 in accordance with that ratio is determined as the position p1. The moving speed of the focusing lens required to keep an in-focus state can be found from the positional difference between the positions p1 and p0 and the time required for the variator lens to move between the positions z0 and z1.
A case in which the stop position of the variator lens is not limited only to boundaries having stored representative locus data will be described below with reference to FIG. 4. FIG. 4 is a view aiding in explaining a method of interpolating the position of the variator lens. FIG. 4 is an extracted portion of FIG. 3(a) and shows a case in which the variator lens can be stopped at an arbitrary stop position.
In FIG. 4, the vertical and horizontal axes respectively represent the position of the focusing lens and the position of the variator lens. Letting Z0, Z1, . . . , Zk-1, Zk, . . . Zn represent the position of the variator lens, the corresponding positions of the focusing lens for different subject distances, i.e., the representative locus positions (the position of the focusing lens relative to the position of the variator lens) stored in a lens control microcomputer are as follows:
a0, a1, . . . , ak-1, ak, . . . an, PA1 b0, b1, . . . , bk-1, bk, . . . bn.
If it is now assumed that the position of the variator lens is Zx which is not a zoom boundary position and the position of the focusing lens is px, positions ax and bx are obtained as follows: EQU ax=ak-(Zk-Zx)*((ak-ak-1)/(Zk-Zk-1)), (2) EQU bx=bk-(Zk-Zx)*((bk-bk-1)/(Zk-Zk-1)). (3)
Specifically, locus data corresponding to the same subject distance are selected from among four stored representative locus data (ak, ak-1, bk, bk-1 in FIG. 4) and internally divided in accordance with an internal ratio which is obtained from the current position of the variator lens and two adjacent opposite zoom boundary positions (for example, Zk and Zk-1 in FIG. 4), whereby the positions ax and bx can be obtained. Then, the locus data corresponding to the same focal length, which are selected from among the four stored representative locus data (ak, ak-1, bk, bk-1 in FIG. 4), are internally divided as shown by the above equation (1), in accordance with an internal ratio which is obtained from the positions ax, px and bx, whereby the positions pk and pk-1 can be obtained. Furthermore, during zooming from the wide-angle end toward the telephoto end, the moving speed of the focusing lens required to keep an in-focus state can be found from the difference between the target focusing-lens position pk and the current focusing-lens position px and the time required for the variator lens to move from the position Zx to the position Zk.
Furthermore, during zooming from the telephoto end toward the wide-angle end, the moving speed of the focusing lens required to keep an in-focus state can be found from the difference between the target focusing-lens position pk-1 and the current focusing-lens position px and the time required for the variator lens to move from the position Zx to the position Zk-1. The above-described locus tracing method has been devised.
As can be seen from FIG. 2, if the variator lens moves from the telephoto end toward the wide-angle end in the direction in which divergent loci gradually converge, an in-focus state can be maintained by the above-described locus tracing method. However, if the variator lens moves from the wide-angle end toward the telephoto end, it becomes impossible to determine which locus should be traced by the focusing lens which is located at a point on convergent loci, so that an in-focus state cannot be maintained by the above-described locus tracing method.
FIGS. 5(a) and 5(b) are views aiding in explaining one example of a locus tracing method which has previously been devised to solve the above-described problem. In each of FIGS. 5(a) and 5(b), the horizontal axis represents the position of the variator lens, and the vertical axis of FIG. 5(a) represents a peak level (sharpness peak signal) within a vertical synchronizing period of a high-frequency component of a luminance signal, which is an AF evaluation signal, whereas the vertical axis of FIG. 5(b) represents the position of the focusing lens.
In FIGS. 5(a) and 5(b), it is assumed that a locus 604 is an in-focus cam locus to be used for zooming relative to a certain subject. It is also assumed that an in-focus cam locus tracing speed on the wide-angle side of a variator-lens position 606 (Z14) is positive (the focusing lens moves toward its closest-distance end), and that an in-focus cam locus tracing speed on the telephoto side of the position 606 is negative (the focusing lens moves toward its infinity end). If the focusing lens traces the cam locus 604 while maintaining an in-focus state, the sharpness peak signal exhibits the magnitude shown at 601. It is generally known that zooming which maintains an in-focus state exhibits a sharpness peak signal level which has an approximately constant value.
In FIG. 5(b), Vf0 indicates the moving speed of the focusing lens which traces the in-focus cam locus 604 during zooming, and Vf indicates an actual moving speed of the focusing lens. If zooming is performed while varying its speed with respect to the speed Vf0 which traces the locus 604, a zigzag locus like a locus 605 is obtained. In this case, the sharpness peak signal level varies in such a manner that hills and valleys repeatedly occur like a locus 603. The magnitude of the sharpness peak signal 603 reaches its maximum at each position where the loci 604 and 605 cross each other (even-numbered points among Z0, Z1, . . . , Z16), whereas the magnitude of the sharpness peak signal 603 reaches its minimum at each position where the moving-direction vector of the locus 605 switches over (odd-numbered points among Z0, Z1, . . . , Z16). The sharpness peak signal 603 has a minimum value 602, and if the minimum value 602 is set as a level TH1 and the moving-direction vector of the locus 605 is switched over each time the magnitude of the sharpness peak signal 603 becomes equal to the level TH1, the moving direction of the focusing lens after switchover can be set to a direction closer to the locus 604. In other words, each time an image is defocused by the difference between the levels 601 and 602 (TH1) of the sharpness peak signal, if the moving direction and the moving speed of the focusing lens are controlled to decrease the amount of defocusing, it is possible to effect zooming with the amount of defocusing reduced.
By using the above-described method, in the case of zooming from the wide-angle end toward the telephoto end in which convergent cam loci gradually diverge as shown in FIG. 2, even if the in-focus locus tracing speed Vf0 is unknown, it is possible to select a locus capable of preventing the sharpness peak signal level from falling below the minimum value 602 (TH1), i.e., preventing occurrence of not less than a certain amount of defocusing, by repeating a switchover operation like the locus 605 (in accordance with a variation in the sharpness peak signal level) while controlling the moving speed Vf of the focusing lens with respect to the tracing speed (calculated by using p(n+1) obtained from Equation (1)) described above in connection with FIGS. 3(a) and 3(b).
Letting Vf+ and Vf- be a positive correction speed and a negative correction speed, respectively, the moving speed Vf of the focusing lens is determined as: EQU Vf=Vf0+Vf+, (4) EQU Vf=Vf0-Vf+. (5)
At this time, to prevent the correction speeds Vf+ and Vf- from deviating in either correction direction when a focus locus to be traced is selected, the correction speeds Vf+ and Vf- are determined so that the internal angle made by the two direction vectors of the moving speed Vf which are obtained from the above equations (4) and (5) is divided into two equal angles by the direction vector of the in-focus locus tracing speed Vf0. In addition, another method has been devised which is intended to improve the accuracy of selection of a focus locus to be traced by varying the magnitude of a correction speed according to the kind or state of a subject, the focal length and the depth of field. Yet another method has been devised in which a focus locus is traced by using an integral signal which varies sensitively to defocusing.
However, in any of the above-described examples, if the same subject is photographed in its in-focus state while varying the angle of view from the wide-angle side to the telephoto side, a sharpness integral signal (the value obtained by adding together sharpness peak signals which appear within a horizontal synchronizing period, within the interval of a vertical synchronizing period) varies to a great extent according to the angle of view even during the in-focus state. For this reason, it is difficult to determine whether the subject is in focus, merely by using the sharpness integral signal, and it is impossible to determine whether a variation in the sharpness integral signal is due to defocusing or a variation in the subject. Accordingly, if a correction speed is determined by using only the sharpness integral signal, the correction speed becomes excessively large when the sharpness integral signal is small during an in-focus state. This leads to the problem that the state of focusing unstably fluctuates between an in-focus state and an out-of-focus state during zooming, so that the in-focus state cannot be maintained during zooming.